The present invention relates to a semiconductor device, particularly, a device provided with two terminals, and a method of producing the same.
A glass sealed diode as shown in FIG. 1 is known as a semiconductor device provided with two terminals. It is seen that a semiconductor device 1, i.e., a diode, is housed in a glass container 2 which is held between jumets 3, 3. Lead wires 4, 4 are connected to the device 1 via the jumets 3, 3. A semiconductor device of this type comprises a semiconductor substrate provided with at least two semiconductor regions and front and back surface electrodes mounted on the front and back surfaces of the substrate. FIGS. 2-6 collectively show a method of producing a semiconductor device of this type.
In the first step, a semiconductor substrate 5 comprising an N.sup.+ -region and an N-region formed on the N.sup.+ -region is subjected to thermal oxidation to form insulation films 6 on both surfaces of the substrate. Then, an opening 7 is made in the insulation film 6 on the front surface of the substrate, i.e., on the N-region, followed by diffusing a P-type impurity through the opening 7 into the substrate to form a P.sup.+ -region within the N-region, as shown in FIG. 2. Further, the insulation film on the back surface of the substrate 5 is removed by selective etching. As shown in FIG. 3, a thin insulation film 8 is formed on the insulation film 6 on the front surface and on the exposed substrate surface, followed by polishing the N.sup.+ -region on the back side of the substrate 5 to a prescribed extent. The polishing is intended to control the depth of the N.sup.+ -region. In some cases, the thickness of the substrate is reduced to nearly half the original thickness by the polishing treatment.
After the polishing treatment, a nickel layer is formed on the back surface of the substrate by an electroplating method, followed by sintering the nickel layer at about 700.degree. C. for 10 minutes so as to form a sintered nickel layer 9 as shown in FIG. 4. FIG. 4 also shows a front surface electrode 10, which is formed by making a hole in the thin insulation layer 8 positioned on the P.sup.+ -impurity region by selective etching, followed by vapor deposition of a gold alloy and subsequent selective etching of the gold alloy. Then, a silver bumper 11 is formed to cover the front surface electrode 10 as shown in FIG. 5. Further, a nickel layer 12, a gold layer 13, and a silver layer 14 are formed by an electroplating in the order mentioned on the sintered nickel layer 9 so as to form a back surface electrode.
In forming the silver bumper 11, the substrate is subjected to a pretreatment such as a water wash, followed by applying a silver plating to the substrate. An aqueous solution containing silver cyanide, potassium cyanide, potassium carbonate, potassium hydroxide, etc., is used as a plating bath. Naturally, the substrate and a silver plate immersed in the plating bath are connected to the negative and positive electrodes of a power source, respectivey, so as to form a desired silver bumper on the front surface electrode.
It is important to note that, in the manufacturing process of the conventional semiconductor device, the nickel layer plated on the back surface of the substrate is subjected to a heat treatment in order to form the sintered nickel layer which permits to improvement on the bonding strength of the back surface electrode to the substrate. What should be noted is that the substrate tends to be cracked in the heat treating step. It should also be noted that the substrate thickness is reduced by the polishing applied to the back side of the substrate. As a result, the substrate tends to be easily cracked in the masking step employed several times after the polishing step for forming the front surface electrode as well as in the heating step mentioned above. An additional difficulty accompanying the prior art is that the debris generated in the polishing step is attached to the insulating film. It follows that pin holes are formed in the insulating film in the selective etching step, employed for forming the opening in preparation for the formation of the front surface electrode. Naturally, the resultant semiconductor device is rendered low in reliability.